In the electrophotographic photoreceptor used for a copying machine, a laser printer, and an LED printer, in which an electrophotographic system is used, there has so far been involved the problem that a charging performance is inferior and a stability in a repeated use is short when an organic compound is used as a charge generating material. Particularly in recent years, a process in which the surface of a conductive substrate is roughened by various means is applied in a photoreceptor for the laser printer in order to prevent the generation of an interference fringe. In this case, there used to be involved the problems that the coating defects such as a resist spot and a convex portion formed by foreign matter generate when a charge generating layer is coated on the roughened substrate surface and that the local injection of a charge from a substrate generates a black spot and a blank area on an image.
It is generally known as the means for solving these problems to provide a subbing layer between a conductive substrate and a charge generating layer.
There are known as the material for forming the subbing layer, a thermoplastic resin such as polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl methyl ether, polyamide, thermoplastic polyester, a phenoxy resin, casein, gelatin, and cellulose nitrate, and a thermosetting resin such as polyimide, polyethyleneimine, an epoxy resin, a melamine resin, a phenol resin, and a polyurethane resin.
However, in the case where these resins are used to form the subbing layer, the increase in a layer thickness to improve a charging performance, a coating performance, and a controlling performance for an image quality defect causes the reduction of a sensitivity and the increase in a residual potential in a repeated use. Meanwhile, in the case where the layer thickness is decreased in order to improve these characteristics, there is involved the problem that the charging performance, the coating performance, and the image quality defect can not sufficiently be controlled.
Further, there has been tried the method in which the organic or inorganic conductive particles are dispersed in a resin in order to avoid the problems caused when the thickness of the subbing layer is increased but an expected performance has not been able to obtain.
On the contrary, it is known as shown in JP-A-61-94057 (the term "JP-A" as used herein means an unexamined published Japanese patent application) that the use of the subbing layer containing an organic metal compound as a main component can control a coating defect and an image quality defect without causing the reduction of a sensitivity and the increase in a residual potential.
In the case where there is applied the method in which the coating solution comprising an organic metal compound and a silane coupling agent is used for coating and drying to form the subbing layer, the hardening degree of the subbing layer is under the control of an environment after coating to a large extent. That is, the subbing layer dried at a lower temperature after coating has the lower hardening degree. Further, it was found that in an electrophotographic photoreceptor comprising a photosensitive layer provided on this subbing layer, a residual potential at an initial stage is high under a low temperature and a low humidity and that a residual potential in a repeated use is markedly increased.
Further, in case of the subbing layer containing only an organic metal compound as a main component, the layer obtained after coating and drying is liable to crack and therefore it is difficult to increase a layer thickness to about 0.3 .mu.m or more. Meanwhile, in the case where a photosensitive layer is formed on the conductive substrate having the surface which is toughened for the measure to prevent an interference fringe as described above, the subbing layer is required to have a layer thickness thick enough to sufficiently cover an irregularity on a substrate surface. However, the subbing layer containing the above organic metal compound as the main component can not meet it.
There is disclosed in JP-A-2-59767, the subbing layer comprising an organic titanium compound, a silane coupling agent and a polyvinyl acetal resin, and the subbing layers having the thickness of 1 to 3 .mu.m are shown in the examples. However, according to the investigations by the present inventors, it has been found that the reduction of a sensitivity particularly under a low temperature and a low humidity and the increase in a residual potential are notable at the added amount of the polyvinyl acetal resin shown there.